To charge your HoloLens, connect the power supply to the charging port using the included Micro USB cable. Then plug the power supply into a power outlet. When the device is charging, the battery indicator will light up in a wave pattern.
When your HoloLens is on, the battery indicator shows the battery level in increments. When only one of the five lights is on, the battery level is below 20 percent. If the battery level is critically low and you try to turn on the device, one light will blink briefly, then go out.
To get an estimate of your current battery level, say "Hey Cortana, how much battery do I have left?"